How to Measure the Thermal Stability of Eudragit®
Standard ASTM E2550-11 describes the method of measuring the thermal stability of materials by means of thermogravimetry. As an example, let us determine the thermal stability of Eudragit® L100-55 (Evonik Industries). To this end, you need a thermobalance to measure the mass loss of your sample during heating until thermal decomposition. The TGA measurement was carried out using the following instrument and parameters:
- Instrument: NETZSCH TG 209 F1 Libra thermobalance coupled to the FT-IR System by Bruker Optics
- Sample: Eudragit® L100-55 (Evonik Industries)
- Sample mass: 7.33 mg
- Crucible: open aluminum oxide
- Temperature program: Heating to 600°C at 10 K/min
- Atmosphere: N2 (40 ml/min)
Thermal Stability With TG-FT-IR Measurements
Figure 1 depicts the mass changes of Eudragit® L100-55 between 40°C and 600°C. The first mass-loss step of 0.8% up to 100°C indicates the release of surface water.
Water release or decomposition start?
The second mass loss of 5.9% at 200°C (DTG peak) is associated with the release of:
- Crystal water;
- CH2 and CH3 molecules (bands in the range 3000-2800 cm–1 and above 1000 cm–1).
(see FT-IR spectrum of the gases released at 206°C in figure 2)
In this case, coupling to the FT-IR System is really important: detection of the CH2 and CH3 molecules in the released gases proves that the mass loss is not only due to the release of water! The Eudragit® sample begins to decompose during this mass-loss step starting at 185°C!
This temperatureof decomposition is related to the thermal stability of the sample.
The Decomposition Goes On…
The peak at 294°C in the DTG curve is associated with yet another step in the decomposition process: the release of carbon dioxide and probably ethanol (figures 3 and 4). This can be explained by the splitting of an ester group off the Eudragit® molecule.
The last and main decomposition step, with a mass loss of 88.5%, occurs at 393°C (DTG peak temperature). The characteristic bands of:
- carbon dioxide
- carbon monoxide (2300 cm-1 to 2100 cm-1)
- ester substance (band at 1749 cm-1)
- parts of the carbon Backbone (bands at 1460 cm-1 and 1380 cm-1)
are detected in the FT-IR-spectrum of the gases released at 393°C (figure 5). It suggests that the ester part C2H5-O-CO-CxHy breaks off from the molecule.
Under the selected conditions (inert atmosphere, heating rate of 10 K/min), the investigated Eudragit® sample starts to decompose at 185°C (onset temperature of the TGA curve). The fact that this is really the start of decomposition is revealed by the occurrence of C-H bonds in addition to crystal water.